This invention relates to polyolefins, and more particularly to improved expanded or foamed compositions comprising propylene polymers. Still more particularly, the invention relates to an apparatus including an improved extrusion die and to a method for the extrusion of rigid or semi-rigid polypropylene foam sheet having improved surface appearance. Polypropylene foam sheet according to the invention is readily thermoformable into shaped articles that are particularly useful in rigid and semi-rigid packaging and in fabricating trays, plates, containers and other articles used in food service applications.
Polystyrene has found wide acceptance for use in food service applications because of its good rigidity and shape retention and, as foam sheet, it is readily molded and thermoformed. However, polystyrene articles suffer from low service temperature, and generally are fragile and lack chemical resistance. The food service and packaging arts have long sought alternative materials that do not have these undesirable characteristics.
Polyolefin resins are widely known for their ease of fabrication and are found in a great variety of applications. Propylene polymers, or polypropylene resins, are particularly noted for their good heat resistance and mechanical properties, and resin formulations based on polypropylene are supplied to meet the demands imposed by a variety of structural and decorative uses in the production of molded parts for appliances, household goods and autos. Impact-modified polypropylene and elastomeric ethylene-propylene copolymers have found application in automotive applications including interior trim as well as in exterior parts such as bumper facia, grill components, rocker panels and the like. Polypropylene resins have the thermal and chemical resistance to withstand exposure to the wide variety of environments encountered in automotive uses, and are easily molded at a cost far below that of metal stamping to provide parts that resist rust and corrosion and are impact resistant, even at low temperature. Considerable effort has been expended in recent years to develop rigid expanded or foamed polyolefin sheet as a replacement for polystyrene foams, particularly for use in food service applications. A number of processes for producing polypropylene foam have been disclosed and are well described in the art, including for example the methods disclosed in U.S. Pat. No. 5,180,571 to J. J. Park, et al and those set forth in the references cited and summarized therein.
At the surface of extruded foam sheet there generally may be found a layer consisting substantially of crystalline polypropylene (PP). This surface layer or skin is important to part appearance and surface hardness. The thickness and crystallinity of the PP surface layer that forms depends in part upon extrusion conditions including die temperatures and cooling rates, and upon annealing. The Park et al patent is directed to the extrusion of polypropylene to provide foam sheet with a smooth surface skin and a uniform cell structure. According to Park et al, high melt strength, high melt elasticity polypropylene with a particularized combination of molecular and rheological characteristics including bimodal molecular weight distribution and a minor component that is highly branched is necessary to provide acceptable foam sheet. Patentees provide comparisons showing that low density foam sheet extruded using conventional or generic polypropylene resins, further characterized as polypropylene resins with monomodal molecular weight distributions and an absence of significant branching, generally have roughened sheet surfaces and non-uniform microcellular structure and are unacceptable for commercial use.
The surfaces of extruded polyolefin foam sheet generally lack the smooth, shiny, uniform and substantially unblemished surfaces observed with extruded styrenic foam sheet, particularly including higher density ABS foam sheet. For example, surface roughness is commonly encountered when extruding polyethylene foam sheet, and lack of uniformity in cell structure and distribution at the surface is visually more readily apparent because of the transparent nature of unfilled polyethylene. Sensible surface roughness, that is, roughness that can be sensed tactilely, may be reduced by contacting the lower melt temperature polyethylene sheet with a polishing roll during the extrusion process to give a smooth, more even surface. The surface imperfections that remain are mainly visible density variations and are generally uniformly distributed, providing a textured or marbleized surface appearance that is pleasing and generally acceptable.
Rigid polypropylene foam sheet obtainable from conventional resins by the processes currently known and practiced in the art continues to be somewhat lacking in surface appearance characteristics. Characteristically, polypropylene foam sheet extruded with conventional processes and using conventional or generic polypropylene resins will have regularly-spaced markings in the form of alternating bands or corrugation-like markings extending the length of the sheet in the machine direction. In light, low-density foams obtained from conventional polypropylene resins, particularly soft, flexible foams having densities of 20 lb/ft3 (0.3 g/cm3) and lower, these bands may have the form of a regularly spaced, wave-like or sinusoidal distortion, forming a corrugated sheet. The bands or corrugations become less pronounced for rigid foam sheet and, particularly at higher foam densities, are seen as surface flaws or appearance defects that take the form of linear, valley-like surface depressions along the machine direction.
The surface roughness of sheet extruded using these higher melting resins is more difficult to smooth adequately using a polishing roll. Moreover, the imperfections and visible density variations found in the surfaces of extruded polypropylene foam sheet are often not uniformly distributed over the surface, and are generally quite visible, even for sheet that otherwise is tactilely smooth. In rigid, higher density foams, such as are sought for the production of food service articles, the defects more often appear as a pattern of alternating linear bands of high and low foam density, characterized by readily visible variations in translucence and surface gloss, possibly including surface voids, bubbles, streaks and uneven color. Such flaws may be without significant effect on the mechanical properties of the foam, and generally do not affect the performance of finished goods fabricated from such foam. However, in consumer goods, food packaging, and the like, these visible surface defects and related cosmetic flaws are highly undesirable, thus limiting acceptance of polypropylene foam sheet by the industry.
Coextrusion of multilayer sheets having solid outer skins and a foamed core has been disclosed in the art and is widely used to overcome surface appearance problems encountered in the production of a variety of prior art foam sheet materials including those made from polystyrene and ABS. Foam core sheet, provided with a shiny or glossy unfoamed surface layer formed of the same or another resin, may be improved in resistance to surface abrasion and cuts and have a superior appearance. The more rigid skin serves to stiffen the foam structure, allowing a lighter and thinner structure while attaining maximum bending stiffness. Foam sheet coextrusion processes are well described in the art for use with a variety of resins such as polystyrene and ABS, and methods have been recently disclosed for use in the coextrusion of multilayered foam sheet comprising polyolefins including polypropylene. Coextrusion processes suffer the disadvantage of generally requiring more costly feedblocks, dies and related machinery having a more complicated design, thereby increasing the complexity of the operation and raising cost of producing such foam sheet.
Thus, there continues to be a need for a reliable method for the manufacture of rigid, high density foamed polypropylene sheet comprising conventional generic polypropylene resins with reduced visible surface defects and related cosmetic flaws and having the attractive, defect-free surface appearance necessary for acceptance in the food service and packaging industries.